This application claims the benefit of European patent application No. 99101497.8 filed Jan. 27, 1999, hereby incorporated herein by reference.
This invention relates generally to electrically powered actuators for actuating control mechanisms as valves, chokes or other control mechanisms used in particular for sub-sea oil or gas production systems. Of course such an actuator could also be used in any terrestrial remote or inaccessible location.
U.S. Pat. No. 5,195,721 discloses a fail-safe valve actuator which moves a closure member of the valve between two positions. A separate spring member is arranged for urging the second drive part in a direction to move a closure member to a second position wherein an electrically powered member prevents a first drive part from rotating in an opposite direction and thus holds the closure member in its first position. In response to the loss of the supply of electrical power to the rotating prevention member, the closure member is moved by the spring member to its other position. The rotation preventing means comprises a number of sleeves, gears and pinions and also an electric motor connected by the sleeves, gears and pinions to the drive parts.
For releasing the rotation preventing means, a disc is lifted by a compressed spring and by de-energizing a solenoid, a threaded member is no longer in engagement with a groove in one of the sleeves and a wrapped spring is correspondingly no longer tightened around the sleeves, so that one of the sleeves may be rotated with respect to the other whereby the two driving parts can move relative to one another.
Accordingly, the valve actuator of U.S. Pat. No. 5,195,721 is of a quite complicated construction with a plurality of parts wherein a separate spring must be provided for forcing the rotatable part back in position and whereby the rotation preventing means uses the electric motor in its turned-off mode to create a torque and transmit same to the actuating member to prevent any rotation thereof.
For actuating the control mechanism, an actuating member of the actuator is axially moved and in one position, the control mechanism is, for example, switched on and in another position of the actuating member is switched off. Such actuators are arranged within a housing to protect the device against outer influences at the corresponding terrestrial or aquatic location. Within the housing, an electric motor is arranged for rotating a first rotatable part and a second rotatable part both in engagement with one another. The two rotatable parts form a reversible drive for axially moving the actuating member in a feed direction to the control mechanism for operating it. In its operating position the actuating member is locked by a rotation prevention member and for unlocking the actuating member, a release is provided that permits an axial movement of the actuating member in a direction opposite to the feed direction.
It is, therefore, an object of the invention to provide an actuator of simple construction reliably preventing rotation of the actuating member without loading the electric motor with a torque for preventing such rotation.
This object is solved by an actuator of know construction characterised in that the force acting on the actuating member is externally applied by the control mechanism and the rotation prevention member is arranged between the second rotatable part and the housing to rotationally fix the rotatable part to the housing to prevent rotation in the second direction.
As the force acting on the actuating member is externally applied by the control mechanism, no additional spring or other device arranged within the actuator is needed. Correspondingly, the construction of the actuator is simplified. To hold the actuating member in position where the control mechanism is actuated, it is no longer necessary to use the electric motor and a torque supplied by it in case it is switched-off, but instead the rotation prevention member is supported by the housing to provide a torque in opposite direction to the torque supplied by the force of the control mechanism acting on the actuating member.
A simple embodiment of the reversible drive comprises an internally threaded screw nut as the first rotatable part and an externally threaded screw stem as the second rotatable part.
To lower friction between screw and stem, the screw nut and screw stem may form a ball screwing device with balls therebetween. In such a case it is also possible to use a high speed, low torque motor as the electric motor.
The housing of the actuator can be such that the electric motor is arranged within the housing and in particular at one end thereof opposite to the control mechanism. In such a way the actuator is compact and can be easily handled.
For fixing the electric motor in a simple manner within the housing, it may be fixed to a sleeve-like head member with a longitudinal boring which itself is fixed to the housing. Besides the rotational force transmitting member, a gear box may be arranged between the transmitting member and the electric motor.
A rotational force transmitting member of simple construction is a sleeve in which one end of the ball screw stem is inserted wherein the other end of the ball screw stem extends from the ball screw nut in direction to the actuating member.
As the rotational force transmitting member connects the ball screw nut and the electric motor, the sleeve may be rotationally fixed to the ball screw nut to transmit any rotation of the motor to the nut. The connection between the sleeve and the ball screw nut may be in any way that allows a rigid attachment.
The sleeve may have a closed end at which it is connected to the electric motor or the gear box. It may further have an open end portion protruding from the head member in which the ball screw nut is inserted.
To prevent rotation of the sleeve with respect to the head member in a simple way at least in that direction used for moving the actuating member in direction to the control mechanism, the sleeve may be rotatable with respect to the head member in one direction and rotatably fixed to the head member in the other direction.
This may be realised by a rotation prevention member arranged between the end portion of the sleeve and an end portion of the head member.
A rotation prevention member of simple construction is realized by end portions of the sleeve and the head member of same outer diameter with a wrapped spring as the rotation prevention member arranged at least partially on both of these end portions. This wrapped spring has no influence in case the sleeve is rotated in one direction but tightens in case it is rotated in the other direction.
For rotatably supporting and holding the sleeve in position, radial and/or thrust bearings may be arranged between the end portion of the sleeve and an inner wall of the housing.
To prevent a direct contact of sleeve and housing, a sleeve-like bushing may be arranged between these bearings and the inner wall of the housing wherein this bushing is at least rotationally fixed to the housing. It may further also be fixed to the housing in axial direction, that means in longitudinal direction of the housing or the ball screw stem.
To also rotatably support that part of the ball screw stem protruding from the ball screw nut in direction to the actuating member, a sleeve-like extension member may be fixed with respect to the housing with a boring in which that part of the ball screw stem is rotatably supported.
In one embodiment of the invention, the extension member is fixed to the bushing and may axially extend therefrom in direction to the actuating member.
According to another embodiment of the invention, a further sleeve-like end member may be arranged between the extension member and the actuating member wherein the end member is rotatably supported within the housing and the rotation prevention member is arranged between the end member and the extension member.
A rotation prevention member of simple construction may be realized by the extension member comprising an end sleeve portion with an outer diameter equal to an outer diameter of the end member and by a wrapped spring as rotation prevention member arranged at least partially on the outer surfaces of the end member and extension member.
In case end member and extension member are rotatably fixed to one another and to allow an axial displacement of the ball screw stem relative to end member and extension member, the ball screw stem is axially displaceable and rotationally fixed with respect to the end member.
A simple way to rotationally fix the ball screw stem with respect to the member is to provide a radially extending key member arranged between the ball screw stem and the end member.
This key member may protrude from an inner boring surface of the end member and may be guided in a groove extending longitudinally on an outer surface of the ball screw stem. In this way the ball screw stem and end member are rotationally fixed to one another and a rotation of the ball screw stem may be prevented by the rotation prevention member arranged between the end member and the extension member.
As the extension member is fixed to the bushing or directly to the housing, the end member and, correspondingly, also the ball screw stem are supported by the housing according to the rotation prevention member arranged between the end member and the extension member.
To avoid a direct contact between the ball screw stem and the actuating member, a thrust collar may be arranged between both. By this thrust collar, any relative rotation of ball screw stem and actuating member may be absorbed.
Different embodiments of the actuating member are possible but advantageous is an actuating stem as the actuating member extending in longitudinal direction of the housing and in particular coaxially with the ball screw stem.
In combination with this, the thrust collar may include two parts, one of which is fixed to an end of the ball screw stem and the other one supporting an end of the actuating stem with thrust bearings between the two parts.
The actuator may have a compact shape and may be easily handled in case the housing is tube-like. Moreover, to obtain a simple access for maintenance or the like, the housing may have to end caps fixable at both ends of the housing.
For releasing the wrapped spring as a rotation prevention member, different embodiments are possible. An embodiment of simple construction may be obtained in case the wrapped coil spring has a tang protruding at one end of the spring in essential radial direction and in case the release includes an engagement member releasably engaging the tang for pushing it in circumferential direction of the coil spring to release same and to allow rotation of the actuating stem in the second direction.
A simple and easily operable actuating member for such a tang may be a solenoid as a further part of the release with a plunger movable in direction to the tang. The plunger may directly push the tang to loosen the wrapped spring wherein this plunger is the engagement member.
It is also possible to arrange a cam member as the engagement member between the plunger and the tang which is pivotally supported between an engagement position and a release position wherein the cam member contacts and pushes the tang in engagement position and is spaced from the tang in release position. The cam member is pivoted from release position to engagement position by actuating the solenoid and driving the plunger in direction to the tang.
The cam member may have different shapes adapted for engagement with the plunger and the tang. In one embodiment of the cam member, it has the shape of a sector of a circle with one radius assigned to the tang and the other radius assigned to the plunger wherein the cam member is pivotally supported at an intersection of the two radii opposite to its circumference.
As the tang member radially outwardly extends from the coil or the extension end member, it is advantageous when the cam member is pivotally supported by a pivot axis extending in parallel and outwardly spaced with respect to the longitudinal axis of the ball screw stem or the housing.
For providing a support for the cam member, a tube-like housing may be provided extending between the thrust collar and the extension member wherein the cam member is pivotally supported in a gap provided in a peripheral surface of the tube-like housing.
The solenoid may also be arranged in the housing of the actuator. It is also possible to provide a separate housing or casing for the solenoid radially extending from and releasably fixed to the outer housing of the actuator.
To monitor the actuator and in particular any movement of the actuating stem, it may be recommendable to arrange at least one sensor for detecting the position of the actuating stem within the housing. Such a sensor may be a proximity switch or any other kind of sensor that can at least detect the two extreme end positions of the actuating stem.
As such actuators are used at remote terrestirial or aquatic locations that may be inaccessible, they should have a fail-safe function. This may be easily realized by the present actuator in that the plunger is spring-loaded in direction to the cam member for loosening the wrapped spring by pushing its tang in case of de-energized solenoid to provide such a fail-safe actuator.
To prevent any shocks within the actuator or by operating the control mechanism, an absorbing member may be arranged movable with the actuator stem.
In a simple embodiment, such an absorbing member plate-like surrounds the actuator stem and is fixed thereto.
For obtaining a general maintenance free actuator, a lubricant may be filled in the housing to be supplied to all moving parts within the housing which lubricant may be also used by the absorbing member to damp its movement together with the actuator stem.
As the actuator may be operated under different temperatures, the lubricant may have different volumes. Therefore, a compensator may be connected to the interior of the housing for receiving or supplying lubricant from or to the housing.
As the actuator may be operated under extreme environmental conditions, as for example sub-sea, it is advantageous to seal the housing with respect to these conditions. Accordingly, a number of sealing rings may be provided for at least sealing of the end caps with respect to the housing.
As the actuator may be also operated under explosive conditions, it is also advantageous when an explosion-proof electrical connector is provided in the housing for receiving voltage supply connectable to the electric motor.
Other advantageous features of this invention will be apparent to those skilled in the art from a consideration of this specification, including the attached drawings and dependent claims.